Biochar from residues of anaerobic digestion and its application as electrocatalyst in Zn-air batteries

Background: Biochar, the product obtained by pyrolysis of biomass, is a new eco-friendly material with excellent properties and many promising applications. Among them it can be used as cathode in Zn-air batteries with very satisfactory results Methods: The residue was obtained by anaerobic digestion of a mixture of corn silage (10%), malt (10%), and cattle manure (80%), aiming to biogas production. It was first freeze-dried and then it was subjected to pyrolysis up to 800oC. Significant findings: The biochar was physicochemically characterized. It has moderate specific surface area, sufficient sp2/sp3 ratio and metal and non-metal surface chemical species. The biochar demonstrated satisfactory electrocatalytic performance, both as oxygen reduction and oxygen evolution catalyst. When applied as electrocatalyst in Zn-air batteries it reached an open-circuit potential of 1.45 V, a short-circuit current density of 200 mA cm-2 and maximum power density of around 62 mW cm-2. Its energy density was 927 Wh kg-1, (at 20 mA), and 518 Wh kg-1 (at 100 mA). In a charge-discharge mode at 10 mA, the potential varied between 1.35 and 1.90 V. These data, show that the waste biomass can be used as inexpensive material for Zn-air batteries and offers a useful approach to combine waste management with energy storage.